When two packets collide, the medium in a data network may remain unusable for the duration of transmission of both damaged packets. For packets that are long compared to propagation time, the amount of wasted capacity can be considerable. This waste can be reduced if a station continues to listen to the medium while it is transmitting.
The most commonly used medium access control technique for bus/tree topologies is carrier sense multiple access with collision detection (CSMA/CD). For the Ethernet network, the ANSI/IEEE Standard 802.3 (ISO/IEC 8802-03, 1996) requires a media access control (MAC) engine in a CSMA/CD communication station to monitor the medium for traffic. If a collision is detected during transmission, the transmitted frame is aborted, and a brief jamming sequence is produced to assure that all stations in the network know that there has been a collision. After the jamming sequence is transmitted, the communication station attempts to transmit the aborted frame again.
In accordance with the ANSI/IEEE Standard 802.3, if a collision is detected after the preamble of a frame is completed, but prior to 512 bits being transmitted, the MAC engine will abort the transmission and append the jam sequence immediately. The jamming sequence is a 32-bit all zeros pattern. The MAC engine will attempt to transmit a frame a total of 16 times.
If a collision is detected after 512 bits have been transmitted, the collision is termed a late collision. In this case, the MAC engine will abort the transmission, and append the jam sequence. No retry attempt will be scheduled on detection of the late collision, and the transmitted message will be discarded.
Thus. the ANSI/IEEE Standard 802.3 establishes a late collision slot time for a collision detection procedure to retry the transmission of a frame if a collision is detected within the late collision slot time, and to discard the frame if a collision is detected outside the late collision slot. However, different physical layer devices (PHY) can have different internal delays for collision detection. As a result, false detection of the late collision can occur, when the collision actually occurs within the late collision slot time.
Thus, it would be desirable to provide a system for adjusting the late collision slot time to prevent the false late collision detection.